The mutation responsible for Huntington's disease (HD) is an abnormal CAG repeat expansion in the HTT gene, which encodes the protein, huntingtin (Htt). Despite HD being a single gene disorder, there is enormous variability in disease onset, severity of symptoms and course of illness. Pathogenesis of HD is largely associated with expression of the mutant Htt (mHtt) protein making it the most significant molecular target for disease modifying therapies. Several therapeutic approaches aimed at its expression, processing, or turnover are under intense development and already in clinical trials. Hence, measures of Htt/mHtt have broad potential as a biomarker, provided that levels can be reliably measured in a surrogate tissue/fluid. Since non-invasive methods to quantify Htt in the CNS do not exist, measuring the amount of Htt in peripheral cells represents an important step in biomarker discovery for HD. Although Htt/mHtt has been successfully measured in CSF and blood, the collection of these fluids is invasive, and repeated sampling can be cumbersome. Alternatively, we propose to use saliva as a source for measuring Htt/mHtt. As a biospecimen, saliva is an improvement over blood sampling due it its ease of collection, non-invasive nature, lack of need for trained personnel and it is amenable to repeated sampling. Importantly, many molecules found in the human brain that are relevant to CNS function have also been identified in saliva, such as growth factors, neurotransmitters and even pathological proteins, such as phosphorylated tau and ?- synuclein. In our preliminary studies, we found that salivary levels of total Htt (both normal and mutant forms) can be reproducibly and reliably measured in human saliva using Western blotting and ELISA methodologies. We further showed that total Htt protein levels were elevated in saliva samples from HD patients compared to control individuals and that salivary levels of total Htt were correlated with motor symptoms in HD patients. In the current proposal, we propose to extend these studies to measure mutant-only forms of the Htt protein and to identify the predominant forms of Htt in saliva, with an overall goal to characterize the utility of salivary Htt/mHtt for biomarker purposes in HD (a single Aim 1). These Aim 1 studies will include: 1). identifying the major forms of Htt/mHtt protein in saliva; 2). quantifying Htt/mHtt in saliva from HD patients using immunoassays; 3). correlating salivary Htt/mHtt levels with clinical data; and 4). determining the association between salivary Htt/mHtt levels and those measured from other sources, including serum, white blood cell fractions and buccal cells. Salivary readouts of Htt/mHtt could be especially useful in assessing the effects of systemically delivered HTT-lowering therapies, or detecting peripheral effects of centrally delivered therapies.